As a disk device, a magnetic disk drive is known, which comprises a housing including a base and a top cover and accommodating therein a rotatable magnetic disk and an actuator which supports a magnetic head. Further, such a method of improving the performance of a disk drive has been proposed, that the housing is sealed with a low-density gas such as helium so as to reduce the rotation resistance of the magnetic disk and the magnetic head.
In such a magnetic disk drive, the top cover is jointed to the base of the housing by laser welding to form an enclosed housing and increase the airtightness of the housing. The laser welding is carried out on along an entire outer circumference of the top cover. Here, in order to obtain high airtightness, it is necessary to maintain a stable welding quality all around the circumference.